[Paper Review] Multi-object spectroscopy of low redshift EIS clusters. III. Properties of optically selected clusters

L. F. Olsen, Christophe Benoıst+3 more|ArXiv.org|Feb 25, 2005

Astronomy and Astrophysical Research29 references7 citations

TL;DR

This study confirms 20 of 21 low-redshift (z ≈ 0.2) ESO Imaging Survey (EIS) cluster candidates using multi-object spectroscopy, revealing velocity dispersions from 130 to 1200 km/s and identifying 53% with a significant red sequence. The results confirm the matched-filter detection method’s reliability and show that red sequence colours align with passive stellar evolution models, supporting the physical reality of the detected systems as galaxy groups and clusters.

ABSTRACT

We have carried out an investigation of the properties of low redshift EIS clusters using both spectroscopy and imaging data. We present new redshifts for 738 galaxies in 21 ESO Imaging Survey (EIS) Cluster fields. We use the ``gap''-technique to search for significant overdensities in redshift space and to identify groups/clusters of galaxies corresponding to the original EIS matched filter cluster candidates. In this way we spectroscopically confirm 20 of the 21 cluster candidates with a matched-filter estimated redshift z_MF=0.2. We have now obtained spectroscopic redshifts for 34 EIS cluster candidates with z_MF=0.2 (see also Hansen et al., 2002; Olsen et al., 2003). Of those we spectroscopically confirm 32 with redshifts ranging from z=0.064 to 0.283. We find that: 1) the velocity dispersions of the systems range from sigma_v<=130km/s to sigma_v=1200km/s, typical of galaxy groups to rich clusters; 2) richnesses corresponding to Abell classes R<=1; and 3) concentration indices ranging from C=0.2 to C=1.2. From the analysis of the colours of the galaxy populations we find that 53% of the spectroscopically confirmed systems have a ``significant'' red sequence. These systems are on average richer and have higher velocity dispersions. We find that the colour of the red sequence galaxies matches passive stellar evolution predictions.

Motivation & Objective

• To spectroscopically confirm low-redshift EIS cluster candidates (zMF ≈ 0.2) identified via the matched-filter technique.
• To characterize the dynamical and photometric properties of the confirmed systems, including velocity dispersion, richness, and concentration.
• To assess the presence and properties of red sequences in the galaxy populations to evaluate the systems’ physical coherence and evolutionary state.
• To test whether the observed red sequence colours match predictions from passive stellar population models.
• To evaluate the impact of projection effects on system property measurements and the reliability of the matched-filter detection method.

Proposed method

• Acquired multi-object spectroscopy for 738 galaxies in 21 EIS cluster fields using the Danish 1.5-m telescope at La Silla.
• Applied the 'gap' technique to identify significant overdensities in redshift space, defining redshift groups as potential clusters.
• Associated spectroscopically identified groups with original EIS matched-filter cluster candidates based on spatial coincidence and redshift consistency.
• Calculated velocity dispersions, richness (Abell R), and concentration indices (C) from spectroscopic and photometric data.
• Constructed colour-magnitude diagrams to detect red sequences and compared observed colours with passive evolution models (Bruzual & Charlot 1993) and non-evolving templates.
• Quantified scatter in red sequence colours relative to passive evolution models to assess consistency.

Experimental results

Research questions

• RQ1 Are the 21 low-redshift EIS cluster candidates (zMF ≈ 0.2) physically real clusters, or are they spurious detections?
• RQ2 What is the range of dynamical properties (velocity dispersion, richness, concentration) among the confirmed systems?
• RQ3 How many of the confirmed systems exhibit a red sequence, and are their colours consistent with passive stellar population evolution?
• RQ4 To what extent do projection effects from foreground or background structures bias the measured global properties of the systems?
• RQ5 How well do the observed red sequence colours match theoretical predictions for passively evolving elliptical galaxies?

Key findings

• 20 out of 21 EIS cluster candidates with zMF = 0.2 were spectroscopically confirmed, with redshifts ranging from z = 0.064 to z = 0.283, in excellent agreement with matched-filter estimates.
• Velocity dispersions span a broad range from 130 km/s (group-like) to 1200 km/s (rich cluster-like), indicating diverse dynamical states.
• 53% of the spectroscopically confirmed systems (17 out of 32) show a significant red sequence, with colours consistent with passive stellar evolution models.
• The scatter in red sequence colours around the passive evolution model is 0.14, comparable to the bin width used in colour measurements.
• Systems with detected red sequences tend to have higher velocity dispersions and greater richness than those without, but no significant correlation with concentration index.
• Projection effects may affect up to 13 out of 32 systems, potentially biasing velocity dispersion and richness estimates, and explaining the weak correlation between these two properties.